Polymer 2 Flashcards
General properties of Polymers
-properties related to amorphous structere
-properties related to molcular bond
-properties related to mechanical behavure
properties related to amorphous structere
1- light in weight- low densitiy
2-translucent cuz its semi-cristline (if 100 amorphous then its transparent)
3-either less or more britile
4-have glasstransition temperature “ melting range”. [Tg] short
arrangment of atoms.
Proprties related to mouluclar bond 2ry between the polymar chain ( weak nd physical in nature)
1- low meting range of tm
2-high coofitent of thermal expantion
3-electricl and vhemical insulator
4-absorb h2o
5- low modules of elastic - flexible
6- low strenth - weak
7- low hardness - soft
- Mechanical behavior
1-The stress strain relationship of polymer compared to
ceramic & metal.
Metal
More flex
Weaker
More britle
Less ductile
More britle and less ductile
Ceramic
More flex
Weaker
Less britle
More ductile
- Mechanical behaviour
2- deformition
Initial strees on the polymer
Chain unfolding
Chain sliding
Further stress
Bond stretching
Bond breaking
- Mechanical behaviour
3-crazing
Voids in the polymer will open during suvice and form a fine cracks prepindicular on the force
Reasons for denture base crazing
1- due to mechanical stress during denture expantion
2-thermal stress mismatching
3-solvent action
Factors affecting the properties of
polymers:
1- structeral factors
2-additive factors
Structural factors
1-degree of polymraizition :
number of repting mers or units in the polymer chain
M.W. Of the polymer : m.w of mers x d.p
Chain legneth
The distribution of chain length is more important
than the molecular weight or degree of
polymerization. This explains the different strength
properties of two specimens of the same polymer
with the same M.W.
Chain lenght
Increasing the m.w and d.p will increase the chain length
Incres 2ry bond and enlargment will increase the strength
Structural factors: copolymrizition
Poly chain has more than one type of mers
1- random
2-graft
3-block
Effect:
-Graft and block improve the impact strenght proprteis
-talio maker
Structural factors: Crystallinity
The poly is usualy amophus with short chain , they are semi\crystalline with regions of crystals
surrounded by amorphous areas.
Crystal area = folded and extnded polymer chain
Amorphous = uncoordinated chain
Factors preventing high degree of
crystallization of polymers:
1- size of blundet groub
2- rapid cooling
3- high degree of polymrizition and molecular weight
4-network structer ( crosslinked)
Effect of crystallinity:
Increase:
-strenght
-density
-hardness
-tg
Decrease:
-water sorbtion transparency
